(a) Field of the Invention
The present invention relates to a coaxial type signal line and the fabricating method thereof. More specifically, the present invention relates to a coaxial type signal line and its fabricating method in order to accomplish the removal of signal interference generated in a radio frequency (RF) electrical system and to reduce the overall dimensions of the whole system.
(b) Description of the Related Art
A conventional RF signal system utilizes coplanar and micro strip type transmission lines.
The coplanar type transmission line is flat and easy to manufacture. The characteristic impedance of a coplanar type transmission line is determined by the area of the signal line and a gap from the ground. This characteristic impedance depends on the type and quality of the substrate. This coplanar type is the transmission line generally used when a system or circuit is implemented on a semiconductor.
The characteristic impedance of the micro strip type transmission line is determined by a height between the grounding of the lower ground and the signal line and the area of the signal line. This type of transmission line is generally used in an implementation of a system using a printed circuit board (PCB). Recently, it has also been used in semiconductors systems.
Both types of these two signal lines are not shielded, that is, are open, and during signal transmission, radiative losses occur. Additionally, the open areas of these signal lines generate interference for other signal lines. To solve this problem, metal is either deposited to shield the upper part of these two types of signal lines or a lower substrate is etched. However, these efforts are only partially effective as well as being difficult to implement, and the fundamental losses caused by signal interference and radiation are not prevented.
Recently, since the frequencies used in systems are becoming higher, and RF transmitting and receiving systems must be implemented in very small sized PCB, signal interference is becoming a significant factor influencing system performance.
Therefore, in order to improve signal interference and loss features of an open type signal line, there was a need to develop a closed type signal line.
FIG. 1 is a closed signal line structure disclosed in the Japanese patent publication No. 91-211870.
As shown by the conventional closed signal line structure of FIG. 1, a first ground conductor 2 is formed on a semiconductor substrate 1, and a first dielectric layer 3 is formed on the first ground conductor 2. A signal line 4 is formed around the center of the first dielectric layer 3, and a second dielectric layer 5 is formed on the first dielectric layer 3 and the signal line 4. A second ground conductor 6 surrounds the first dielectric layer 3 and the second dielectric layer 5, and is connected to the first ground conductor 2.
According to this structure, since the signal line 4 is electrically shielded by the first and second ground conductors 2 and 6, interference with adjacent signal lines can be reduced.
However, according to the conventional signal line structure as shown in FIG. 1, since the signal line 4 is not positioned on the surface of the semiconductor substrate, but is positioned at a predetermined distance away from the surface of the semiconductor substrate, it is not easy for the signal line 4 to be connected with other signal lines on the semiconductor substrate. Further, although the signal line 4 is connected to the other signal line on the surface of the semiconductor substrate, the signal lines are easily disconnected. Hence, the conventional signal line structure can only be used in the packaging of the signal lines, and the structure cannot be directly applied to the manufacture of integrated circuits (ICs).
Additionally, since the signal line 4 in the conventional signal line structure is formed on a dielectric material, as the frequency transmitted via the signal line 4 becomes higher, the losses increase.
It is an object of the present invention to provide a coaxial type signal line and its manufacturing method to solve the problems of signal interference and the connection of signal lines associated with a RF electrical system.
In one aspect of the present invention, a coaxial type signal line comprises a substrate including a groove; a first ground line formed on a surface of the groove and on a plain surface of the substrate; a first dielectric layer formed on the first ground line formed on the surface of the groove; a signal line, formed on the first dielectric layer, that transmits signals; a second dielectric layer, formed on the signal line and the first dielectric layer, that surrounds the signal line; and a second ground line, formed on the second dielectric layer and the first ground line that electrically shields the signal line.
In another aspect of the present invention, a coaxial type signal line comprises a substrate including a groove; a first ground line formed on a surface of the groove and on a plain surface of the substrate; a supporter formed on the first ground line formed on the surface of the groove; a signal line, formed on the supporter, that transmits signals; and a second ground line connected to the first ground line formed on the plain surface of the substrate, that encloses a space that surrounds the signal line.
In a still another aspect of the present invention, a method for manufacturing a coaxial type signal line comprises the steps of (a) forming a groove on a substrate; (b) forming a first ground line on a surface of the groove and a plain surface of the substrate; (c) forming a first dielectric layer, including dielectric material, on the first ground line formed on the surface of the groove; (d) forming a signal line on the first dielectric layer, the signal line for transmitting signals; (e) forming a second dielectric layer, including dielectric material, on the signal line and the first dielectric layer; and (f) forming a second ground line on the first ground line and the second dielectric layer.
In a still further aspect of the present invention, a method for manufacturing a coaxial type signal line comprises the steps of (a) forming a groove on a substrate; (b) forming a first ground line on a surface of the groove and a plain surface of the substrate, (c) forming a supporter on the first ground line formed on the surface of the groove; (d) forming a first photoresist on the first ground line and the supporter formed on the surface of the groove, and removing the first photoresist on the supporter so as to expose the supporter; (e) forming a signal line on the exposed supporter, the signal line for transmitting signals; (f) forming a second photoresist on the signal line and the first photoresist; (g) forming a second ground line on the first ground line and the second photoresist; and (h) removing the first and second photoresist within the first and second ground lines.
The above step (d) comprises the steps of forming a first photoresist on the first ground line and the supporter formed on the surface of the groove and the plain surface of the substrate; and removing the first photoresist on the supporter so as to expose the supporter.
The above step (e) comprises the steps of forming sacrificial metal for coating on the first photoresist and the exposed supporter; coating a third photoresist on the sacrificial metal, and removing the third photoresist on the supporter so as to expose the sacrificial metal formed on the supporter, and forming a signal line for signal transmission on the exposed sacrificial metal by a coating process.
The above step (g) comprises the steps of removing the third photoresist and the sacrificial metal; smoothing the first photoresist and exposing the first ground line on the plain surface of the substrate; and forming the second photoresist on the signal line and the first photoresist.